Today, highly efficient energy conversion systems based on fuel cells are the focus of global research and development activities, because compared to systems based on conventional energy sources they show considerable advantages with regards to efficiency and exhaust emissions. In addition to the construction of stationary power plants, their greatest potential is in automotive construction.
The effective principle of PEM (Polymer Electrolyte Membrane) fuel cells is based on a controlled electrochemical reaction of hydrogen and oxygen utilizing the electric energy emitted. These fuel cells comprise an anode chamber and a cathode chamber, separated by an electrolyte membrane. At the anode side of the membrane, hydrogen molecules are catalytically split and move in the form of protons into the cathode chamber, where they form water with the oxygen ions. This way, fuel cells generate electrochemical energy. Preferably air may serve as the oxygen supplier at the cathode side, which allows a particularly economic operation.
However, in air-operated PEM-fuel cells a problem arises, for example caused by aging, and the gas permeability of the membrane accompanied here due to the increased permeation of nitrogen from the air into the anode chamber. This is visible by the accumulation of nitrogen at the side of the anode. A critical situation develops when micro-fractures form in the membrane, for example macroscopic cracks. In case of a macroscopic crack of the membrane overheating occurs, due to direct catalytic incineration of hydrogen at the leakage site, which directly leads to the destruction of the membrane and thus to the fuel cell failing to function.
Furthermore, another type of malfunction is possible, which may occur by abnormal concentrations of water vapors of the anode gas. The transportation of ions through the PEM is only possible with sufficient moisture. Dehydration due to insufficient concentration of water vapors of the supply gas leads under load to a local increase of the current density in the still sufficiently moistened membrane areas, and thus may also lead to damages up to a failure of the fuel cell due to overload. Therefore it is important for the operation of air-operated PEM-fuel cells to reliably prove small amounts of nitrogen in the anode chamber, in addition to various concentrations of water vapor. This way, on the one hand a current permeability of the membrane can be monitored, which allows conclusions about the aging process of the membrane and thus leakages connected thereto. On the other hand it is possible to monitor the current moisture level. This way the fuel cell processes can be reliably controlled and the life of the fuel cell is increased, which increases the maintenance intervals required.
A method to measure nitrogen in a gas is known from DE 602 23 961 T2, in which trace amounts of nitrogen in krypton or xenon gas are measured using a gas discharge tube in a semiconductor-production process. The intensity of the light generated by the discharge in the gas discharge tube is measured via a spectrophotometer. Here, the underlying gas is restricted to noble gases, such as krypton and xenon.
A sensor is known from U.S. Pat. No. 5,570,179 for the spectroscopic analysis of gaseous mixtures. This sensor operates with a silent electric discharge. This silent electric discharge could not be maintained in a typical gas pressure of approximately one bar, thus the system is not suitable for use in fuel cell systems.
U.S. Pat. No. 5,168,323 describes a device for determining contaminants in gases. This device is particularly used for determining contaminants in noble gases, such as helium. Here, no indication is given for the determination of nitrogen concentrations in an atmosphere similar to the one of fuel cells. The option of a direct determination of the concentration of water vapors in a gaseous mixture is not mentioned, either.
Methods to determine a leakage of fuel gases in a fuel gas system are known from DE 10 2006 001 778 T5 and DE 11 2006 002 060 T5, which are based on estimates and or evaluations of nitrogen concentrations. Both methods are not suitable for the online analysis of fuel cells during operation and omit the use of spectroscopic methods for proof.